The energy levels in atoms and ions are the key to the production and detection of light. Energy levels or "shells"exist for electrons in atoms and molecules. The colors of dyes and other compounds results from electron jumps between these shells or levels. The colors of fireworks result from jumps of electrons from one shell to another. Observations of light emitted by the elements is also evidence for the existence of shells, subshsells and energy levels. The kinds of light that interact with atoms indicate the energy differences between shells and energy levels in the quantum theory model of the atom. Typically the valence electrons are the ones involved in these jumps.

The "quantum" theory was proposed more than 90 years ago, and has been confirmed by thousands of experiments. Science and education has failed to clearly describe the energy level concept to almost four generations of citizens. This experiment is an exercise aimed at throwing a little more light on the subject. ( Don't laugh too hard at the joke.)

Atoms have two kinds of states; a ground state and an excited state. The ground state is the state in which the electrons in the atom are in their lowest energy levels possible (atoms naturally are in the ground state). This means the electrons have the lowest possible values for "n" the principal quantum number.

Specific quantized amounts of energy are needed to excite an electron in an atom and produce an excited state. The animation shows the opposite of excitation. It shows how the excited hydrogen atom with an electron in the n = 3 shell can release energy. If the electron in hydrogen only drops to the n = 2 shell the energy matches a pulse of red light.

Note the size of the electron cloud in the excited atom changes when the electron moves from shell to shell. The size of the atom decreases in volume when the electron goes from the n=3 shell to the n = 2 shell. On average the electrons are closer to the nucleus for lower values of "n". The electron cloud is related to the most probable distance between the nucleus and the electron. The most probable distance increases with increasing "n" value. The excited electron is still "in" the atom even in an excited state. The valence electron will only escape the atom if the electron is given an amount of energy equal to the ionization energy for that atom. (Be sure to view Animation) return to top of page

Energy can be added to atoms many different ways. It can be in the form of light, an electric discharge or heat. This added or extra energy is emitted when the excited electrons in the atoms give off light and fall back to lower shells. The light emitted has wavelengths and colors that depend on the amount of energy originally absorbed by the atoms. Usually each individual excited atom will emit one type of light. Since we have billions and billions of atoms we get billions of excitations and emissions. return to top of page

Not all atoms in a sample will absorb or be excited exactly the same. For example in hydrogen the ground state has the electron in the n= 1 shell. The electron in some hydrogen atoms may be excited into the n = 2 level. Other hydrogen atoms can have the electron excited into the n = 4 shell.

Different elements emit different emission spectra when they are excited because each type of element has a unique energy shell or energy level system. Each element has a different set of emission colors because they have different energy level spacings. We will see the emission spectra or pattern of wavelengths (atomic spectra) emitted by six different elements in this lab. We will then identify an unknown element by comparing the color of the unknown with the flame color of our knowns.

You need to know that white light is the combination of all colors of the spectrum.

Each color has a characteristic wavelength. The wavelength is the distance between the beginning and end of a complete cycle of the light wave. All colors of light travel at the same speed, 3.0 x 10⁸ meters/ second. The animation shows how a prism separates photons of red light from photons of blue light. The photons of different colors fall in different positions on the color spectrum. The position is determined by the wavelength. (Be sure to view Animation)return to top of page

return to top of page

Arizona State University has a nice page on Light at this URL.

http://acept.la.asu.edu/PiN/rdg/readings.shtml

Colorado State University physics department has an animation of the H atom at this URL

http://www.colorado.edu/physics/2000/quantumzone/lines2.html

Definitions of terms and properties of light are at this site return to top of page

http://www.aspsky.org/html/tnl/35/atom.html

The speed of light frequency and wavelength are related by the formula

c = l n

where c is the speed of light 3.00 x 10⁸ meters/ second.

n is the frequency of the light wave in cycles per second

Lambda, l, is the wavelength in meters/cycle.

The frequency,n , equals the number of cycles that pass an observer in a second. These individual cycles each carry a specific small amount of energy. The particles of light are called photons. The energy of a light photon or particle is different for each color. If you have ever seen a Star Trek movie you may have heard of "photon torpedoes" . The science fiction writers suggest that bundles of light energy could be harnessed and used as projectiles.return to top of page

We can calculate the energy of a photon or wave packet using the Planck energy relation shown here. Planck won a Nobel Prize for this work. This relation allows us to tell how much energy a single photon can carry.

return to top of page

Red light has longer wavelength and is lower in energy than blue light. The wavelength of red light corresponds to the range of 700 to 600 nanometers, (7000 Ångstrom or 0.0000007 meters).

Blue light has shorter wavelength in the range of 400 nm (4000 Ångstrom or 0.00000004 meter,

1 Å = 1 x 10 ^-10 m = 0.0000000001 meter = 1 x 10^-1 nanometer). return to top of page

Spectroscopy is the analysis of light spectra and the way in which light interacts with matter. When light is analyzed it is commonly separated into its component colors. The light source is directed on a slit and the "beam" of light is separated using a prism or grating.

The reason that the images are lines is that the light from the lamp is focused on a narrow slit. The illustration shows the separation of a light beam into its component colors. return to top of page

This produces an image of the slit which has the shape of a line. The resulting beam of light can be broken into the color spectrum. or into its components of the spectrum emitted by the atom. You can see the specific colors emitted by the light source. A white light source will give a spectrum like the one shown above. return to top of page

The emission spectrum for sodium shows only two colors in the visible color range. The two colors are yellow and have wavelengths of about 590 nm. The continuous spectrum and calibration scale is shown to give approximate wavelength values.

return to top of page

A CD-ROM mirrored surface behaves like a grating or prism. If you look at the surface of the CD-ROM under a light, you can see the color spectrum.

Street lights are sodium vapor lamps in many communities. These lamps have an orange yellow tint. You can see from the emission spectrum why the sodium vapor lamps would appear yellow and not white. These lamps consume less energy than the older blue colored mercury vapor lamps. Mercury vapor lamps have been sold in hardware stores for yard lighting.

One of the odd things that we sometimes notice is that colored things have a different appearance in natural daylight than they do under mercury vapor or sodium vapor lamps. This is reasonable because the daylight includes all of the wavelengths of white light and the vapor lights only emit a few specific colors that can be reflected into our eye off of any illuminated article.

Example calculations:return to top of page

Frequency

The energy of a photon or single pulse of light energy can be calculated using the Planck energy relation

Note: A common household 100 watt light bulb uses 100 joules every second. Even though this energy is not converted completely into light, we can make a rough assumption that it is. If this energy is emitted as light ( really much is disipated as heat) it means 3.28 x 10²⁰ photons (330,000,000,000,000,000,000 photons) would have to be emitted every second to transfer the 100 joules in a second. The numbers are huge. By way of comparison the population of the wor;ld is only 8,000,000,000 (eight billion). The atoms and their electrons are constantly changing energy. The electrons are going through ceaseless energy level jumps.

Experimental Procedurereturn to top of page

Part 1 Flame tests and identification of an unknown metal.

Observe and record the color of the flame for each metal ion. Remember the metal ions are paired with a nonmetal ion in an ionic formula unit. The electrical charges have to add to zero. The metal ions are converted to atoms in the flame and then excited by the heat from the Bunsen burner flame. The nonmetal ions, anions, do not get converted to atoms and do not and emit visible light like the metals do.

Repeat procedure for each known. Record the color observed for the unknown and use the color match to identify the metal atom that is the produced from the cation in the unknown.

Metal ion

return to procedure

Observed Flame color

barium

click for flame test

_________________________

calcium

click for flame test

_________________________

sodium

click for flame test

_________________________

rubidium

click for flame test

_________________________

potassium

click for flame test

_________________________

lithium

click for flame test

_________________________

Students in Dr. Volland's class should send in lab report by equiz